Many wireless communication utilize direct sequence spread spectrum to communicate information. The codes used to spread a signal are typically pseudo random codes. A receiver typically recovers the underlying information by correlating the spreading code with a locally generated code.
A receiver can sometimes utilize a time offset associated with the codes to establish a timing reference that can be used to perform position location. Determining position based on timing established from pseudo random spread signals is performed in various position location systems.
The Global Positioning System (GPS) navigation system employs satellites that are in orbits around the Earth. Any user of GPS, anywhere on Earth, can derive precise navigation information including 3-dimensional position, velocity and time of day. The GPS system includes at least 24 satellites that are deployed in medium earth orbits with radii of 26,600 kilometers in six planes inclined at 55° with respect to the equator and spaced 60° with respect to one another. Four satellites are equally spaced within each of the six orbit paths. Position measurements using GPS are based on measurements of propagation delay times of GPS signals broadcast from the orbiting satellites to a GPS receiver. Normally, reception of signals from 4 satellites is required for precise position determination in 4 dimensions (latitude, longitude, altitude, and time). Once the receiver has measured the respective signal propagation delays, the range to each satellite is calculated by multiplying each delay by the speed of light. Then, the location and time are found by solving a set of four equations with four unknowns incorporating the measured ranges and the known locations of the satellites. The precise capabilities of the GPS system are maintained by means of on-board atomic clocks for each satellite and by ground tracking stations that continuously monitor and correct satellite clock and orbit parameters.
Each GPS satellite transmits several direct-sequence-coded spread spectrum signals over multiple carriers in the L-band. An L1 signal is positioned at a carrier frequency of 1.57542 GHz, an L2 signal is positioned at 1.2276 GHz, an L3 signal is positioned at 1.38105 GHz, an L4 signal is positioned at 1.379913 GHz, and an L5 signal is positioned at 1.17645 GHz. Civilian accessible signals transmitted on the L1, L2, and L5 frequencies are of most interest in the context of position location and navigation.
The L1 signal consists of two phase-shift keyed (PSK) spread spectrum signals modulated in phase quadrature. The P-code signal (P for precise), and the C/A-code signal (C/A for coarse/acquisition). The L2 signal contains the P-code signal as well as an L2C civilian accessible signal. The P and C/A codes are repetitive pseudo-random sequences of bits (also referred to as “chips”) that are modulated onto the carriers. The L2C signal contains two distinct PRN sequences. A first signal, referred to as a CM signal (for Civilian Moderate length code) is 10,230 bits in length, repeating every 20 milliseconds. A CL (for Civilian Long length code) is 767,250 bits, repeating every 1500 milliseconds.
The clock-like nature of these codes is utilized by the receiver in making time delay measurements. The codes for each satellite are unique, allowing the receiver to distinguish which satellite transmitted a given code, even though they are all at the same carrier frequency. Also modulated onto each C/A carrier is a 50 bit/sec data stream that contains information about system status and satellite orbit parameters, which are needed for the navigation calculations. The CM signal is modulated with a 25 bit/sec navigation message with forward error correction, whereas the CL signal is a non-data sequence. The P-code signals are encrypted, and are not generally available for commercial and private users. The C/A signal is available to all users.
The operations performed in a GPS receiver are for the most part typical of those performed in any direct-sequence spread spectrum receiver. The spreading effect of the pseudo-random code modulation must be removed from each signal by multiplying it by a time-aligned, locally-generated copy of the code, in a process known as despreading. Since the appropriate time alignment, or code delay, is unlikely to be known at receiver start-up, it must be determined by searching during the initial “acquisition” phase of a GPS receiver's operation. Once determined, proper code time-alignment is maintained during the “tracking” phase of GPS receiver operation.
Once the received C/A signal is despread, each signal consists of a 50 bit/sec PSK signal at an intermediate carrier frequency. The exact frequency of this signal is uncertain due to the Doppler effect caused by relative movement between satellite and terminal unit, and to local receiver GPS clock reference error. During initial signal acquisition this Doppler frequency must also be searched for, since it is usually unknown prior to acquisition. Once the Doppler frequency is approximately determined, carrier demodulation proceeds.
After carrier demodulation, data bit timing is derived by a bit synchronization loop and the data stream is finally detected. A navigation calculation may be undertaken once the signals from four satellites have been acquired and locked onto, the necessary time delay and Doppler measurements have been made, and a sufficient number of data bits (enough to determine the GPS time reference and orbit parameters) have been received.
One drawback of the GPS system for location determination is the long time needed for the initial signal acquisition phase. As mentioned above, before the four satellite signals can be tracked they must be searched for in a two-dimensional search “space”, whose dimensions are code-phase delay, and Doppler frequency shift. Typically, if there is no prior knowledge of a signal's location within this search space, as would be the case after a receiver “cold start”, a large number of code delays (about 2000) and Doppler frequencies (about 15) must be searched for each satellite that is to be acquired and tracked. Thus, for each signal, up to 30,000 locations in the search space must be examined. Typically these locations are examined one-at-a-time sequentially, a process which can take 5 to 10 minutes. The acquisition time is further lengthened if the identities (i.e., PN-codes) of the four satellites within view of the receiving antenna are unknown.
In the case where a GPS receiver has already acquired the satellite signals and is then in tracking mode, the position determination process is virtually instantaneous. However, in the routine use of wireless terminals, users turn the power on and quickly begin operation. This may be the case when an emergency communication is intended. In such situations, the time delay associated with a 5 to 10 minute GPS satellite signal acquisition cold-start by a GPS/wireless terminal unit before a position fix can be obtained limits the response time of the system.
Thus, a need remains for a system and method for decreasing the time required to acquire GPS satellite signals and render a position fix in a GPS/wireless terminal unit.